In order to keep pace with the increasing demand for more and more circuits in satellite communications systems, it has been found desirable for these systems to employ higher carrier frequencies, such as the 11/14 GHz and 20/30 GHz bands. While use of these higher frequency bands has permitted the available bandwidth to be increased, thereby allowing a greater number of channels to be carried, as might be expected, certain disadvantages are also presented when the carrier frequency is raised. One such serious disadvantage is that precipitation acts to greatly attenuate both the received and transmitted signals. Furthermore, it has been found experimentally that this attenuation becomes greater as the carrier frequency increases. It can then be seen that as carrier frequencies are inevitably increased the precipitation attenuation problem will become more acute. One obvious solution to this problem is to increase the signal strength so as to simply overpower the attenuation. This solution is not entirely feasible, however, because in the down-link case a satellite transponder does not have unlimited power available, and in the up-link case there are finite limitations on the size of the transmitter amplifiers. Also, although increasing the power may serve to alleviate the precipitation attentuation problem, error correction may still be necessary. Conversely, error detection/correction standing alone is not always sufficient to provide accurate and reliable communications. A better approach to solving this attenuation problem is to use some form of diversity combined with error control.
The general concept of diversity is well-known in the field of communications and may be simply stated as the technique of transmitting and/or receiving the same message more than once in order to avoid simultaneous fades of the identical message-carrying signals due to propagation anomalies in the communications path. Diversity provides a fail-safe type of system, which may become relatively error free, by relying upon the principle of redundancy. Since the same message will be transmitted more than once, it may then be possible to receive and correlate the messages to determine if an error has occurred. And, of course, the greater the redundancy the greater the probability that the correct message will be received. The systems which have been designed to embody this diversity technique have been many and varied. For example, there is Time Diversity where the same message is transmitted by a single transmitter more than once; there is Frequency Diversity where the same message is transmitted in different frequency bands; and there is Polarization Diversity where the same message is transmitted at different polarization senses. Once the multiple identical messages have been transmitted and received by way of one of the diversity schemes, the signals must be combined or correlated in some manner at the receiver. There are various approaches which also may be taken to achieve this combining function. For example, there is Post-Detection Combining, Pre-Detection Combining, Maximum Ratio Combining and Equal-Gain Combining. Each method of combining also involves an individual approach to error detection and correction which usually is not directly related to the diversity schemes being employed.